What is asphalt and how is it made?

Asphalt is a mixture of aggregates and asphalt cement (liquid asphalt). The asphalt cement is the glue that holds the aggregates together. The mixture is scientifically engineered and heated over 300 degrees so it can be placed, compacted, and finished to a smooth surface.

What are the benefits of using asphalt?

Asphalt covers 94% of the roads in the United States. Asphalt requires less construction time and creates a quieter ride that reduces driver fatigue. Asphalt is smooth, durable, and skid resistant.

How about recycling asphalt?

Asphalt pavement is the most recycled material in America. 100% of an asphalt pavement can be picked up, remixed with a portion of fresh materials and used again. The quality of recycled asphalt pavement is as good as brand new pavement. The asphalt paving industry recycles more than 70 million tons of asphalt pavements every year, more than the combined total of glass, paper, plastic, and aluminium combined.

How is asphalt pavement built?

The asphalt pavement is built in layers. The first step is to remove topsoil and compact the earth. Next, a rock base is placed and compacted. Then, two or more layers of hot mix asphalt are placed and compacted.

How safe is asphalt pavement?

Asphalt pavements can be designed so that water drains through the surface layer of the pavement, thus reducing splash and tire spray, and increases tire-road contact during wet weather. The asphalt pavement retains heat better than other materials, so ice doesn't form as quickly and melts faster.

What happens at an asphalt plant?

At the asphalt plant aggregates are dried and heated, then mixed and covered with asphalt cement.

What is the basic technology of asphalt pavement?

Asphalt pavement is usually approximately 95 percent aggregates, which could be stone, sand, or gravel, and 5 percent asphalt cement as a binder. The binder is a product of oil refining and acts to glue the aggregates together.

Is that heated?

The aggregate and asphalt are heated, combined with the recycled material and mixed together. Then we load the hot pavement material into trucks and take it out to the site.

Can you design an asphalt pavement that is safer during rainstorms?

One of the great things about asphalt pavement surfaces is it can be engineered for different types of conditions. If the project is in an area which receives significant rainfall, then a best practice is to use an open-graded pavement. This open-graded, or porous pavement, will allow the water to drain out through the pavement structure. Because the water can drain off the surface, the risk of hydroplaning is reduced which improves safety. Another important safety feature of an open-graded asphalt surface is that it reduces the splash and spray that is common with other types of pavement. These pavements may also be used to reduce road noise in urban areas.

What is SMA Mix?

SMA, or stone matrix asphalt, mix is a very tough and durable mix. In addition to providing a long-lasting pavement surface, it is used to reduce pavement noise. Due to the expense and harshness of the SMA mixes, these mixes are typically used on interstates and other high volume roadways.

Are asphalt pavements quieter than other types of pavement?

There is considerable research indicating asphalt pavements tend to be quieter than concrete pavements overall.

How have asphalt plants changed over time?

Many modern asphalt plants are a permanent part of the communities which they serve. The plants need to be near where roads are built, because the paving material has to be delivered to the paving site while it is still hot. Portable asphalt plants are normally used in more remote areas, and they can temporarily be set up near a paving site. Whether asphalt plants are on permanent or temporary sites, they are environmentally friendly, and are good neighbours within their communities.

What equipment is used at a typical asphalt paving site?

A milling machine is typically used to remove the surface material from an existing roadway. That material is loaded into a truck and transported to the asphalt plant for recycling. A broom is then utilized to clean the milled surface, followed by a distributor truck which applies a tack coat. The tack coat provides adhesion between the new pavement and the existing surface. A dump truck delivers HMA paving material from the asphalt plant to the paver or the material transfer vehicle (MTV). The paver lays a smooth mat. Then a series of rollers follow the paver to compact the freshly-placed asphalt material. These compactors may be vibratory or static steel wheel rollers or rubber tire rollers. The material transfer vehicle is gaining popularity with contractors and agencies. Basically, it keeps the paver moving at a constant speed to improve smoothness and remixes the asphalt material to keep the mix from segregating and to help insure a uniform temperature.

How have asphalt plants changed over time?

Many modern asphalt plants are a permanent part of the communities which they serve. The plants need to be near where roads are built, because the paving material has to be delivered to the paving site while it is still hot. Portable asphalt plants are normally used in more remote areas, and they can temporarily be set up near a paving site. Whether asphalt plants are on permanent or temporary sites, they are environmentally friendly, and are good neighbours within their communities.

What equipment is used at a typical asphalt paving site?

A milling machine is typically used to remove the surface material from an existing roadway. That material is loaded into a truck and transported to the asphalt plant for recycling. A broom is then utilized to clean the milled surface, followed by a distributor truck which applies a tack coat. The tack coat provides adhesion between the new pavement and the existing surface. A dump truck delivers HMA paving material from the asphalt plant to the paver or the material transfer vehicle (MTV). The paver lays a smooth mat. Then a series of rollers follow the paver to compact the freshly-placed asphalt material. These compactors may be vibratory or static steel wheel rollers or rubber tire rollers. The material transfer vehicle is gaining popularity with contractors and agencies. Basically, it keeps the paver moving at a constant speed to improve smoothness and remixes the asphalt material to keep the mix from segregating and to help insure a uniform temperature.

How thick do you lay the asphalt?

All that is engineered. You have to look at what kind of stresses will be placed on the pavement, trucks vs. cars, and other factors such as soil conditions and climate. It also depends on what materials you intend to use in the asphalt and what materials might be present in the lower layers of the pavement. All these considerations are combined to obtain the pavement thickness. The types of materials used in the HMA are also chosen for specific applications. For example, an intersection may require a different mix from a city street. An intersection requires more strength because cars and trucks are going to be standing on it, as well as starting, stopping, and turning. This puts a great deal of stress on the pavement surface. You have to use a stiffer mix. Asphalt pavement is very much an engineered product.

What is an overlay?

An overlay is a new layer of asphalt applied to existing asphalt. The new layer is typically 1" to 2" deep.

What causes a pothole in asphalt?

A pothole is caused by water penetrating the asphalt, usually through a crack, and damaging the stone base during the freezing and thawing cycles of water. This damage will eventually reach the pavement surface resulting in a pothole.

Everyone is interested in extending the lifetime of highway pavements. What are the advantages of asphalt in this regard?

Well-designed and well-built asphalt pavements last many years. There are a number of case studies which support this conclusion. These pavements have only required maintenance and periodic replacement of their surface layer. The National Center for Asphalt Technology in Auburn, Alabama, is currently conducting experiments on its test track directed toward improving asphalt pavement performance. They are putting 10 million ESALS (Equivalent Single Axle Loads) on the pavement in two years. They are using a wide range of asphalt mixes, including conventional, Superpave, Stone Matrix Asphalt, and Open Graded Friction Courses. The tests show that asphalt mixes which are well-designed and well-built will give very good service. They will last a long, long time.

Can you design an asphalt highway in different ways?

Pavement design is continually evolving. Empirical processes were developed 40 to 60 years ago. These involved observations of how pavements interacted with soils, climates and levels of traffic, and then basing the pavement thickness on these observations. While empirical methods are still widely used, the industry is moving toward mechanistic-empirical procedures. In these newer design methods, the pavement is treated like a structure such as a building or bridge. The reactions of the pavement to different traffic loads are modelled mathematically, giving designers more information concerning options on materials and design factors. This could result in more efficient utilization of materials to obtain long-lasting pavements.

What is the difference between asphalt and concrete?

Asphalt contains aggregate held together by oils and applied hot. Concrete contains cement, aggregate and water and is applied cold.

What is “Perpetual Pavement?

Asphalt pavements can last a lifetime because it's possible to maintain them just with milling and overlays, and the deeper portion of the pavement structure remains sound. That's often referred to as Perpetual Pavement. If you design a pavement correctly for the amount of traffic it will have to stand up to, you can have actually a permanent pavement structure. With new heavy-duty surface pavements, it is possible for overlays to last more than 20 years.

Are concrete pavements more expensive to build than asphalt?

The initial cost of asphalt pavement construction is usually less than concrete. But, in addition to construction cost, an increasingly important factor is the traffic delay cost incurred by the public during construction or rehabilitation. You can't close down a busy road and spend weeks repairing it without costing businesses and individuals potentially millions of dollars. With asphalt, the construction and rehabilitation operations frequently are performed at night.

What is “Warm Mix Asphalt?

Warm-mix asphalt technologies allow the producers of asphalt pavement material to lower the temperatures at which the material is mixed and placed on the road. Reductions of 50° to 100° Fahrenheit have been documented. Such drastic reductions have the obvious benefits of cutting fuel consumption and decreasing the production of greenhouse gases. In addition, engineering benefits include better compaction on the road, the ability to haul paving mix for longer distances, and extending the paving season by being able to pave at lower temperatures.

Department of Transportation Standard Specifications

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